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In any event, the question of whether there are "missing links" between ancient primates and humans is now rather pointless, given the thousands of transitional fossils that have been discovered, representing numerous different species. The only real questions here are how all of these ancient fossils fit in the family tree, which are in the direct line of human ancestry, and how many more species are still to be found. Here is a reconstruction of this family tree as understood in 2007, due to Ian Tattersall (presented here with permission from Tattersall):

Tattersall's graphic is also shown in [Prothero2007, pg. 337]. Note that the above graphic does not include several very recently discovered species, such as Ardipithecus ramidus and Australopithecus sediba, which are mentioned below.

Creationist writers, who deny that there are any "transitional" fossils between primates and humans (because this would counter their belief in "special creation" of humans), typically deal with prehuman fossils by classifying them as either "ape" or "human." However, in a rather ironic twist of events, these writers have not been able to agree as to which fossils should be classified "ape" or "human." Here is a chart of creationist classification of some prehuman fossil specimens, based on a similar chart in [TalkOrigin2009]:

Specimen	Brain cavity size	Cuozzo1988	Gish1985	Mehlert1996	Bowden1981 Gish1979 Menton1988 Taylor1992	Baker1976 Taylor1995	Lubenow2004 Taylor1996	Line2005
ER 1813	510 cc	Ape	Ape	Ape	Ape	Ape	Ape	Human
Java	940 cc	Ape	Ape	Human	Ape	Ape	Ape	Human
Peking	915-1225 cc	Ape	Ape	Human	Ape	Human	Human	Human
ER 1470	750 cc	Ape	Ape	Ape	Human	Human	Human	Human
ER 3733	850 cc	Ape	Human	Human	Human	Human	Human	Human
WT 15000	880 cc	Ape	Human	Human	Human	Human	Human	Human

In the above table, the column headers denote different creationist publications -- see [Cuozzo1988; Gish1985; Mehlert1996; Bowden1981; Gish1979; Menton1988; Taylor1992; Baker1976; Taylor1995; Lubenow2004; Taylor1996; Line2005]. In the second column, the "cc" figures denote the size of the brain cavity in cubic centimeters.

The utter disagreement in this table as to whether fossils are "ape" or "human" is moot testimony to the fact that there is no clear delineation -- all are related in a family tree. As biologist Kenneth Miller observes, "Ironically, validation of our common ancestry with other primates comes directly from those [creationists] who are most critical of the idea." [Miller2008, pg. 95].

What's more, recent studies continue to identify even more hominin species ("hominin" species means prehuman species that are either in the line of descent that led to humans or at least are closely related to humans). This was driven home with the 2003 discovery of Homo floresiensis, better known as the "hobbit," on the island of Flores in the Indian Ocean north of Australia. This discovery precipitated considerable controversy, which continues to the present day, but the emerging scientific consensus is that the hobbit represents a distinct hominin species, as shown in Tattersall's graphic above, that somehow survived until as recently as 18,000 BCE. Some scientists remain unconvinced [Callaway2009], but most others agree that the case is now closed [Flores2010]. Some additional details on the Homo floresiensis debate are given at Conspiracy.

This wave of discoveries continues unabated, arguably at an accelerating pace. Here are some that have been found just in the past few years (as of 2012):

1. The "Ardi" fossil. In 2009, a team of researchers led by Tim White of U.C. Berkeley published their finding of a rather complete fossil skeleton named Ardipithecus ramidus, or Ardi, which was dated to 4.4 million years ago. This is more than one million years older than Lucy, the famous skeleton of the species Australopithecus afarensis [Wilford2009]. White's team concluded that the skeleton was likely in the path that led to modern humans. However, in May 2010, another team, led by Thure Cerling of the University of Utah, questioned this conclusion, noting that an analysis of soils and silica in the area surrounding the find indicated a tree-savana or bush-savana environment (and thus not a likely habitat for hominins) [Wilford2010]. White's team testily replied that the Cerling team ignored "the totality of the fossil, geological and geochemical evidence" presented it the original papers, including, the abundant presence of fossilized mammals adapted to wooded life. This fact, according to White, established that Ardi lived in "closed habitats," not in open savanna and thus was plausibly a human ancestor [Wilford2010].

2. Denisovan fossil. In March 2010, a team lead by two researchers at the Max Planck Institute for Evolutionary Anthropology in Leipzig announced the discovery of a new hominin species, identified as the result of analyzing mitochondrial DNA (mtDNA) found in a finger and tooth found in the Denisova region of Siberia. This species co-existed with humans until as recently as 50,000 years ago, yet is roughly twice as distant (measured in terms of the time since a common ancestor) from modern humans as Neanderthals. As explained in a Scientific American report [Wong2010a],

   Comparing the order of the genetic "letters" -- or base-pairs, as they are termed -- making up the Denisova [Siberian] mtDNA with the sequences of modern day humans and an early modern human, Krause and his collaborators found that the Denisova mtDNA differed from humans today in nearly twice as many letter positions as Neanderthal mtDNAs do.

   A follow-up study published in December 2010, based on an entire genome sequence of the specimen, found that not only do the Denisovans represent a "sister" species to Neanderthals, but that in fact this race of prehumans evidently interbred with Southeast Asian humans, since the genomes of modern-day New Guinea natives contain 4.8% Denisovan DNA [Zimmer2010a]. What's more, an additional study published in August 2011 noted that interbreeding with Neanderthals and Denisovans actually boosted human immunity to viruses [McGrath2011]. For additional discussion of DNA evidence for evolution, see DNA.

3. New South African fossil. One of the most dramatic discoveries of the past few years is the April 2010 announcement, by Lee Berger of South Africa, of a new hominin species, named "Australopithecus sediba," based on some unusually well-preserved South African fossils, including one near-complete skull, which were discovered in 2008 by Lee Berger and his nine-year-old son. These specimens lived approximately 1.977 million years ago, during the era when both australopithecines and early species of Homo existed. Berger and his colleagues of the University of the Witwatersrand in Johannesburg, suggest that this species descended from Australopithecus africanus, but is ancestral to Homo erectus, which in turn is widely believed to be the ancestor of Homo sapiens [Wong2010b; Wade2011c]. However, as with the Ardi skeleton mentioned above, other paleontologists are not so sure. Meave Leakey of the National Museums of Kenya, for example, does not think that this species is ancestral to Homo or has anything to do with Homo. Similarly, anthropologist Berrnard Wood places little credence in Berger's claim that their specimen is a direct ancestor of humans. Additional studies will be required to resolve this question. These controversies are summarized in a nicely written feature article in the April 2012 Scientific American [Wong2012].

4. Lucy's great-grandfather. In June 2010, a team of anthropologists led by Yohannes Haile-Selassie, Director of Anthropology at the Cleveland Museum of Natural History, found fossilized bones of a specimen they nick-named "Kadanuumuu," which is Afar for "Big Man." The specimen is 3.58 million years old, which is more than 400,000 years older than the famous "Lucy" specimen, and thus has been dubbed by some "Lucy's great-grandfather." Among other things, this specimen walked upright on two feet, thus establishing that bipedalism was fully established among ancestral species at least 3.58 million years ago [Dalton2010].

5. Qesem Cave (Israel) fossil. In December 2010, a team of researchers led by Israel Hershkovitz of Tel Aviv University published results on the analysis of some teeth found at the Qesem Cave site in Israel. These teeth were dated to between 200,000 BCE and 400,000 BCE. Researchers who analyzed these teeth, using techniques such as CT scans and X-rays, found that they are very similar to human teeth. Thus, if substantiated, these may be some of the oldest truly human artifacts. However, they also exhibit some affinities with Neanderthal specimens, and thus may represent some common ancestor (such as the Denisovan species) prior to the time when humans and Neanderthals split [SD2010h; Hershkovitz2010].

We should celebrate, not resist, these findings, since they underscore how singular and successful our species has been. What specific features (anatomical and mental) did humans develop that others did not? Which of these features led to our survival and ultimate dominance? These are active areas of scientific research, and hopefully some answers will be uncovered in the coming years.

For additional discussion, see Fossils and God of the gaps.

References